// 1st obs: we reject the test case when there exist heights of toys differ by more than 1.
// 2nd obs: If at one rod, the number of high toys on both sides of that rod is positive, the answer will be -1
// 2st obs: From 2nd obs, we only need to care about the side with positive number of high toys. We swap if needed, then recurse on the rod with difference.
